The invention relates to a discharge lamp according to the precharacterizing clause of claim 1.
The term xe2x80x9cdischarge lampxe2x80x9d here covers sources of electromagnetic radiation based on gas discharges. The spectrum of the radiation can in this case cover both the visible range and the UV (ultraviolet)/VUV (vacuum ultraviolet) range, as well as the IR (infrared) range. Furthermore, a phosphor layer may also be provided for converting invisible radiation into visible radiation.
The case in point deals with discharge lamps having so-called dielectrically impededed electrodes. The dielectrically impeded electrodes are typically produced in the form of thin metal strips, at least a part of which is arranged on the inner wall of the discharge vessel. At least a part of these inner-wall electrodes is fully concealed from the interior of the discharge vessel by a dielectric barrier layer.
If only electrodes of a single polarityxe2x80x94preferably the anodesxe2x80x94are covered with a dielectric barrier layer, then in preferable unipolar operation a so-called unilaterally dielectrically impeded discharge is formed. However, if all the electrodes, i.e. both polarities, are covered with a dielectric barrier layer, then both in unipolar and bipolar operation a bilaterally dielectrically impeded discharge is formed.
On the dielectric barrier layer, and in general on all other parts of the inner wall of the discharge vessel as well, at least one other functional layer is applied, e.g. a layer of a phosphor or phosphor blend and/or one or more layers which reflect visible radiation (light) and/or UV radiation. The purpose of the reflective layer is to send out visible light in a controlled way, i.e. only in a particular preferred direction of the lamp.
There are no particular restrictions on the geometrical shape of the discharge vessel. For example, tubular or flat discharge vessels are commonplace, the latter being amongst other things suitable as so-called flat lamps for the back-lighting of liquid crystal displays (LCDs).
The starting materials for both the reflective and the phosphor layer or layers are initially in the form of powders with a suitable grain size. These powders are then applied as a suspension, usually mixed with an organic binder, with a defined layer thickness to the inner wall of the lamp or to the previously applied other functional layers, e.g. electrodes and dielectric barrier layer. The thickness of the reflective or phosphor layer is, controlled through the viscosity of the suspension, adapted to the respective coating process. After drying and heating, the reflective and/or phosphor layers are in the form of porous powder layer or layers.
Besides the phosphor layer thickness, the uniformity of the reflective and/or phosphor layer as well as its mechanical bonding strength, which decreases as the layer thickness increases, are also important conditions for obtaining optimum conversion of UV light to visible light.
The dielectric barrier layer usually consists of glass frits, preferably lead borosilicate glass (Pbxe2x80x94Bxe2x80x94Sixe2x80x94O).
In the case of flat lamps, whose discharge vessels respectively consist of an essentially plane base glass, a similar front glass and, optionally, a frame, the base glass is provided with a so-called solder edge which likewise consists of a glass frit, preferably PbBxe2x80x94Sixe2x80x94O. The purpose of this solder edge is to bond the components of the discharge vessel (base glass, frame, front glass) in vacuum-tight fashion during the assembly process. This assembly process involves carrying out a thermal treatment in which the solder edge xe2x80x9cmeltsxe2x80x9d to a defined degree, i.e. reaches a defined viscosity.
The reflective and/or phosphor layers are usually applied before this assembly process. Because of this, in addition to the solder edge, the dielectric barrier layer also returns to lower viscosity at the assembly temperature. The overlying porous reflective and/or phosphor layers are hence in turn torn by the xe2x80x9cmovementxe2x80x9d in the dielectric barrier layer (xe2x80x9cice-floe formationxe2x80x9d). The reason for this is that the porous layers have no cohesion and hence cannot join in with this movement without damage, but instead tear and/or even sink partly into the dielectric barrier layer. The uniformity of the reflective and phosphor layer is hence compromised, which causes light losses. Furthermore, these xe2x80x9cice floesxe2x80x9d are clearly identifiable during lamp operation as light-density non-uniformity, for example on the luminous side of a flat lamp.
The object of the present invention is to avoid the disadvantages mentioned above and to provide a discharge lamp according to the precharacterizing cause of claim 1 which has a phosphor and/or reflective layer improved in terms of homogeneity.
This object is achieved by the characterizing features of claim 1. Particularly advantageous refinements are described in the dependent claims.
According to the invention, that layer which is arranged essentially directly underneath the phosphor or reflective layer of the discharge lamp consists of a glass solder whose viscosity variation as a function of temperature is irreversible. This feature is described in more detail below. For the sake of simplicity, this layer will also be referred to below as the xe2x80x9csupportingxe2x80x9d layer or xe2x80x9canti-ice-floe layerxe2x80x9d.
In this context, essentially directly underneath the phosphor or reflective layer of the discharge lamp means that as far as possible there should be no other layer between the xe2x80x9csupportingxe2x80x9d layer and the porous phosphor or reflective layer, or at most only a very thin one. The maximum allowable thickness for such an additional layer is dictated by the condition that, when the lamp is heated (heating up, assembly process etc.) the porous phosphor or reflective layer arranged directly above must not be able to tear as a result of excessive xe2x80x9cmovementxe2x80x9d because of the softening of the additional layer. Depending on its make-up and composition, the thickness of any additional layer should not exceed 100 xcexcm, preferably 50 xcexcm, typically 10 xcexcm, ideally 5 xcexcm. The xe2x80x9csupportingxe2x80x9d layer is, however, preferably arranged directly underneath the phosphor or reflective layer, i.e. without any additional layer between the xe2x80x9csupportingxe2x80x9d layer and the phosphor or reflective layer.
This xe2x80x9csupportingxe2x80x9d layer (xe2x80x9canti-ice-floe layerxe2x80x9d) may be formed either by the actual barrier layer acting as a dielectric impediment for the discharge, or by an interlayer arranged between the dielectric barrier layer, on the one hand, and the reflective and/or phosphor layer, on the other.
This interlayer should cover at least all of the dielectric barrier layer, and may even be applied xe2x80x9cfull-surfacexe2x80x9d. For the effect according to the invention, it has been found to be sufficient if the thickness of this xe2x80x9csupportingxe2x80x9d interlayer is of the order of about 10 xcexcm or more. The system, typically in paste form, is applied using standard methods such as spraying, dispensing, roller application, screen or stencil printing, etc.
The dielectric barrier layer can be applied both in strip form to the individual electrodes (for unilateral and bilateral dielectric impediment) andxe2x80x94in the case of bilaterally dielectrically impeded dischargexe2x80x94xe2x80x9cfull-surfacexe2x80x9d by means of a single continuous barrier layer which covers all of the inner-wall electrodes. The selection of the suitable thickness for the barrier layer is essentially dictated by physical discharge requirements and is typically of the order of 10 xcexcm to several hundred xcexcm, in particular between 50 xcexcm and 200 the xcexcm, typically between 80 xcexcm and 180 xcexcm. Furthermorexe2x80x94in the case of bilaterally dielectrically impeded dischargexe2x80x94the thickness of the barrier layer(s) for the anodes or cathodes may also be chosen to be different. Preferably, in unipolar pulse operation (W094/23442), the barrier layer for the anodes is thicker than that for the cathodes, although the layer thicknesses may also be equal.
The advantage of the first solution, i.e. the dielectric barrier layer is at the same time designed as the xe2x80x9csupportingxe2x80x9d layer (xe2x80x9canti-ice-floe layerxe2x80x9d), is essentially that no additional fabrication or printing step is necessary. On the other hand, the solution with the additional interlayer gives an additional degree of freedom for rational material selection for the dielectric barrier layer, especially in terms of the discharge-affecting dielectric as well as electrical properties.
For clearer understanding of the invention, the behaviour of the glass solders customarily used as a supporting glass layer for the porous layers will be explained first. Normally, hence also in the case of the Pbxe2x80x94Bxe2x80x94Sixe2x80x94O glasses, the viscosity decreases as the temperature increases. This behaviour is reproducible as long as the temperature has not been so high that devitrification has already taken place. The term reproducible means that the temperature range in which the glass softens with defined viscosity is virtually constant even under repetition, i.e. in each case after corresponding prior cooling.
Conversely, the glass solders proposed according to the invention do not exhibit this behaviour. Instead, their viscosity variation as a function of temperature is irreversible. In this case, the viscosity does in fact decrease initially as the temperature increases.
Subsequently, howeverxe2x80x94even with further increasing temperaturexe2x80x94an increase in viscosity once more takes place.
This variation in viscosity as a function of temperature is actually exhibited, in particular, by per se known crystallizing glass solders, the use of which as a layer arranged directly underneath the phosphor or reflective layer of the discharge lamp is proposed according to the invention. The aforementioned viscosity increase at constant or even increasing temperature is caused in crystallizing glass solders by the onset of the crystallization process. Using a defined temperature profile, the crystal growth as well as the phase composition and the crystallite size can also be controlled. The so-called sintered glass ceramic obtained in this way is distinguished in that, during a subsequent thermal treatment, it does not start to soften until higher temperatures, typically temperatures about 50-100xc2x0 C. or more higher.
This meets the requirement of obtaining a xe2x80x9csupportivexe2x80x9d layer which is solid at the assembly temperature, i.e. more highly viscous, on which the porous layers can be printed. Through the use of such sintered glass ceramic layers, continuous reflective and/or phosphor layers are obtained, in particular after the assembly process. Bismuth borosilicate glass (Bixe2x80x94Bxe2x80x94Sixe2x80x94O) has proved to be a particularly suitable crystallizing glass solder. Examples of other suitable crystallizing glass solders include zinc bismuth borosilicate glass (Znxe2x80x94Bixe2x80x94Bxe2x80x94Sixe2x80x94O) and zinc borosilicate glass (Znxe2x80x94Bxe2x80x94Sixe2x80x94O).
Good results have also been obtained with certain composite solders with similar viscosity/temperature behaviour.